Kodak Indicator Stop should work just fine as long as the indicator dye has no effect on your prints. It shouldn't, since it has no effect on normally developed prints, but I don't do that much sepia toning, thus the disclaimer.

Assuming that the Kodak Indicator Stop concentrate is 90%, you can just dilute 1+30 and get very close to (just under) 3%. Adjust as needed if the solution is too weak.